Embodiments of the disclosure relate to high pass filters for microwave systems.
For applications where a wideband radio is constantly sensing its environment, strong interferers that lie close to in-band can desensitize the receiver. This is especially true for radio receivers that operate close to high power radar bands. For this reason, microwave filters are often employed to attenuate undesirable signals while preserving signals of interest. For example, one type of receiver is designed to operate >18 GHz. However there are known high power radars below this frequency that can affect the receiver, and for this reason, a microwave high pass filter is necessary.
Typically, in order to achieve high levels of rejection, high order filters are required. Typically, filters are synthesized using lumped element equivalent models from capacitors, inductors, and resistors. The problem with realizing filters at microwave frequencies is that parasitic effects can dominate responses. For example, if a filter were designed with a spiral inductor at microwave frequencies, the inductor may not behave as an ideal inductor, and will have distributed characteristics. For this reason, synthesizing high order microwave filters using lumped elements is difficult.
A high pass filter is unique in that its behavior is much like a waveguide. A rectangular waveguide has a cut-off frequency below which it will not pass signal. A waveguide is compact in size, simple to implement as it is a transmission line modality, and has the lowest loss of all transmission line modalities. Realizing a waveguide on a printed circuit board is the challenge. A rectangular waveguide is a metal structure with set dimension based on the operating frequency.
A prior art high pass filter uses a microstrip with a short-circuited edge. The filter will be described in further detail with reference to FIG. 1. FIG. 1 illustrates a prior art high pass microwave line 100 with a shorted edge.
As shown in the figure, high pass microwave line 100 includes a dielectric substrate 102, an electric wall 104 and a conducting line 106. Conducting line 106 includes a transmission direction portion 108 and a folding portion 110. Electric wall 104 is disposed on a side wall 105 of dielectric substrate 102 so as to extend in a normal direction over dielectric substrate 102. Electric wall 104 provides a short to the bottom of dielectric substrate 102 for conducting line 106.
Conducting line 106 is disposed on dielectric substrate 102 such that transmission direction portion 108 is disposed to provide a direction for transmission of a radio frequency (RF) signal in a direction parallel to electric wall 104 as indicated by dotted arrow 112. Conducting line 106 is additionally disposed on dielectric substrate 102 such that a portion of folding portion 110 extends to electric wall 104, folds down over the thickness of dielectric substrate 102 and further extends to the underside of dielectric substrate 102 as indicated by dotted portion 114.
The width, w, of transmission direction portion 108 is the transmission length of high pass microwave line 100. A distance, x, is from electric wall 104 to the middle of transmission direction portion 108. The length, l, of folding portion 110 in combination with x tune the frequency of high pass microwave line 100. So changes to either l or x will change the cut off frequency of high pass microwave line 100.
FIG. 2 illustrates a graph 200 of the band-pass characteristics of the prior art high pass microwave line 100 with a shorted edge of FIG. 1. As shown in FIG. 2, graph 200 has a y-axis 202, an x-axis 204 and a plurality of sample that follow function 206. Y-axis 202 is in decibels and corresponds to the power transfer of high pass microwave line 100. X-axis 204 is in GHz and corresponds to the frequency of the transmitted signal.
As can be seen in the figure, below the cut-off frequency of 8 GHz, the insertion loss is large, and above the cut-off frequency, the insertion loss is small. The insertion loss in the pass band is primarily due to radiation loss of the microstrip transmission line modality. Nevertheless, the roll-off of high pass microwave line 100, as noted by function 206 at about 8 GHz indicates that high pass microwave line 100 is an effective high pass filter.
Because conducting line 106 folds around dielectric substrate 102 and then electric wall 104 is disposed thereon, fabrication of high pass microwave line 100 is complicated. Further, it is very hard to provide an acceptable, clean connection between electric wall 104 and folding portion 110. When the connection has imperfections, the signal to noise ratio of high pass microwave line 100 decreases. Further, the structure of high pass microwave line 100 does not lend to easy incorporation into system on chip devices.
What is needed is a high pass filter that is useful in microwave applications for system on chip devices.